A vacuum brake power booster of this type is disclosed in DE-OS 39 15 219, for example. To prevent the effective surface at the valve member, to which the pneumatic pressure differential is applied, from increasing by expansion during operation, an annular component is provided which embraces the valve member and limits the radial movability of the rolling fold of the valve member towards the outside.
The effect of a force component, which is caused during evacuation of the vacuum chamber due to the pressure differential between the vacuum pressure and the atmospheric pressure, on the above-mentioned effective surface on the valve member is disadvantageous, which force component acts in the actuating direction and tends to close the sealing seat (vacuum sealing seat) provided on the control housing. This force component must be overcome, in particular during the return movement of the movable wall, in order to open the vacuum sealing seat. Therefore, the spring, which biases the second (atmospheric) sealing seat in opposition to the actuating direction, has to be rated accordingly. However, the result of this measure is an increased response force.
Therefore, an objective of the present invention is to indicate measures which permit improving or optimizing the ratio between the resetting force, acting on the second sealing seat, and the response force of the vacuum brake power booster.
According to the present invention, this objective is achieved by a means which permits applying the pneumatic pressure, prevailing in the working chamber, to the side of the valve member remote from the sealing seats.
To specify the idea of the present invention, in a favorable aspect of the present invention, wherein the control valve is accommodated in a control housing in which the movable wall is incorporated, the valve member confines a pneumatic chamber in the control housing, to which the pneumatic pressure prevailing in the working chamber may be applied.
In still another aspect of the subject matter of the present invention, a particularly short flow path during evacuation and venting of the pneumatic chamber and a rapid change in pressure in the pneumatic chamber is achieved by the valve member having at least one passage proximate its sealing surface which forms a connection between an annular chamber, confined by the sealing seats and connected with the working chamber, and the pneumatic chamber.
The friction developing during movement of the valve member, according to another favorable aspect of the present invention, may be minimized by the valve member having a radially external and a radially internal rolling fold which, along with a retaining ring retaining the valve member in the control housing, confine the pneumatic chamber.
In another embodiment of the vacuum brake power booster according to the present invention, the overall axial length is shortened, and ease of assembly and manufacture is ensured, by the annularly shaped valve member having a radially external and a radially internal sealing lip which, along with a guide element arranged and sealed in the control housing, confine the pneumatic chamber.
In this arrangement, it is favorable that an annular reinforcing element is arranged on the side of the valve member remote from the sealing seats, and a valve spring bears against the reinforcing element and biases the valve member in the direction of the sealing seats. The reinforcing element, preferably, has a U-shaped cross-section and contains at least one opening opposite to the passage of the valve member.
In another variant, the assembly of the control unit is considerably simplified by the reinforcing element being positioned between the rolling folds. Further, this ensures a proper support of the radially internal rolling fold which permits a determined, desired roll-off diameter. To this end, the reinforcing element may have an L-shaped cross-section.
In still another advantageous variant of embodiment of the present invention, the pneumatic chamber is sealed by sealing beads which are respectively adjacent to the external and the internal rolling folds and in sealing abutment on the control housing or the retaining ring. The reliability in operation of the vacuum brake power booster according to the present invention is enhanced by this effective sealing.
Finally, it is of great significance for producing the desired magnitudes of the resetting force by means of the spring mentioned above that the diameter of the radially internal rolling fold is smaller than the diameter of the second sealing seat, or that it is at most as large as the diameter of the second sealing seat.